3GPP (3rd Generation Partnership Project) has introduced in Release 6 the specification of MBMS (Multimedia Broadcast Multicast Service) as the broadcast function and the multicast function for use in a W-CDMA system.
MBMS, which can send the same data to multiple cells at the same time, provides two data transmission modes, PTM (Point-to-Multipoint) transmission and PTP (Point-to-Point) transmission.
The PTM transmission, a transmission mode in which data is sent to all terminals in a cell via IRL (Radio Link), uses the common channel SCCPCH (Secondary Common Control Physical Channel) that is set in each cell as the physical channel. The PTM transmission, capable of sending data via IRL, can send data using a fixed amount of radio resources regardless of the number of receiving terminals, but requires the transmission power high enough to cover the entire cell. This means that, for terminals near the cell boundary, data is sent at an excessive power level.
On the other hand, the PTP transmission, a transmission mode in which data is sent to one terminal via IRL, uses the high-speed physical downlink common channel HS-PDSCH (High Speed—Physical Downlink Shared Channel) or the individual physical channel DPCH (Downlink Physical Channel) as the physical channel. HS-PDSCH, a data channel used for the high-speed packet transmission method HSDPA (High Speed Downlink Packet Access), uses AMCS (Adaptive Modulation and Coding Schemes) that changes the transmission rate according to the reception quality. This data channel can send at a rate higher than that of DPCH. DPCH uses Inner loop Power Control that changes the transmission power according to the reception quality. Therefore, the PTP transmission can save radio resources per IRL when the reception quality is good, but consumes a high transmission power when there are many MBMS reception users because the total amount of transmission data increases in proportion to the number of MBMS reception users.
When MBMS is performed, the RNC (Radio Network Controller) checks the MBMS reception users and counts the number of MBMS reception users for each cell. This is called “counting”.
A cell to which a mobile station is connected to receive MBMS data is called a serving cell. While receiving MBMS data from the serving cell in the PTM transmission mode via SCCPCH, the mobile station obtains a diversity combining gain by receiving the same data at the same time from a neighboring cell in which data is transmitted in the PTM transmission mode. This reception processing is called “combining”.
One of the methods for switching the MBMS transmission mode is a first conventional method in which the number of MBMS reception users (N_count) is compared with a pre-set threshold (N_thr) as shown in FIG. 14. In this first conventional method, the transmission mode is determined as follows. In PTP, a higher transmission power is required as the number of MBMS reception users (N_count) is increased. And so, if N_count is equal to or larger than N_thr, the PTM transmission in which data may be sent to all terminals via IRL is performed (see cell 1 in FIG. 14); conversely, if N_count is smaller than N_thr, the PTP transmission in which data is sent to one terminal via IRL is performed (see cells 2 and 3 in FIG. 14).
Patent Document 1 discloses a method in which, because it is known that the power level in the PTM transmission is sometimes unnecessarily high as described above, the PTM transmission is used at a transmission power value that does not cover the entire cell and, at the same time, the PTP transmission is used for a mobile station that is near the cell boundary and has a low channel quality. In this method, when there are mobile stations in all cell boundaries, both the PTM transmission and the PTP transmission are used at the same time in all cells as shown in FIG. 15.
Patent Document 2 discloses a technology that the same multicast data (called “flow” in the document) is sent at the same time to a sector, from which a data reception request is received, and to the sectors which are the sector's neighboring sectors belonging to the same SHOG (Soft Hand Off Group).
Patent Document 1:
Japanese Patent Kokai Publication No. JP2006-135956A
Patent Document 2:
Japanese Patent Kokai Publication No. JP2006-13826A